There has heretofore been proposed an ultrasonic distance measuring apparatus for measuring a distance to a reflecting body by transmitting ultrasonic waves and receiving reflected waves thereof. According to this ultrasonic distance measuring apparatus, the received signals are passed through a band-pass filter to obtain frequency components only of the transmitted signals, and the obtained signals are detected for their envelope to detect the rising portions (edge portions) of the received signals. Then, the time is measured from the rising portions of the transmitted signals to measure the distance to the reflecting body.
There has further been known a pulse compression method in which modulated signals obtained by modulating a frequency or a phase of pulse signals are transmitted, the reflected waves thereof are received, and a correlation (self-correlation) to the modulated signals is obtained to compress the pulse width of the received signals (see, for example, non-patent document 1: Akira UMEDA and 12 others, “Report of Study Committee on the Facial Detection of a Static Structure by using an Ultrasonic Sensor”, 3-2-6 Ultrasonic Signal Processing, pp. 50-60, [online], Jun. 1, 1992, Foundation Nihon Plant Maintenance Association, retrieved May 24, 2004, <URL: http://www.jipm.or.jp/giken/houkoku/index.htm>).
In the above conventional ultrasonic distance measuring apparatus, the frequency components of transmission signals are obtained through the band-pass filter, making it difficult to remove noise components in the frequency components that pass through the band-pass filter. Therefore, signals having a good SN ratio are not received.
According to the above pulse compression method, further, improvement in the SN ratio can be expected to some extent for the static body (which is not moving) but the pulse cannot be favorably compressed for the received signals reflected by a body that is uniformly moving. This is because when the body is uniformly moving, the frequency of the received signals of reflected waves is shifted by the Doppler effect. When the pulses are compressed, therefore, no correlation is maintained relative to the modulated signals and, as a result, the pulse widths of the received signals are not compressed as desired.